As is well known, a discharge lamp such as a fluorescent lamp emits light when driven by a high-frequency driving voltage generated by an inverter. Discharge lamps of this type are of course used for illumination, and are also used as the light sources of backlights of many liquid crystal display devices in recent years. A discharge lamp is connected, via a connector, to the output terminal on the secondary coil side of a driving transformer formed on the output side of an inverter included in a discharge lamp drive control circuit.
In this case, however, the discharge lamp may not be connected to a connection terminal connected to the output terminal on the secondary coil side of the driving transformer because the discharge lamp and connector are not well connected, or the output terminal on the secondary coil side of the driving transformer may be shortcircuited for some reason. In a case like this, a high voltage of the driving transformer causes discharge, and this may lead to smoke, fire, or the like. In addition, if the discharge lamp itself is broken or old, the output terminal on the secondary coil side of the driving transformer connected to the connector causes a load open circuit state or load short circuit state, and this increases the possibility of smoke, fire, or the like described above.
Accordingly, in order to prevent, e.g., the generation of heat by an abnormal operation such as the load open circuit state or load short circuit state, the conventional discharge lamp drive control circuit has an abnormal operation detecting circuit that detects the open circuit state or short circuit state of the output terminal on the secondary coil side of the driving transformer of the inverter, and stops the operation of the inverter.
The conventionally used abnormal operation detecting circuit has an arrangement in which two comparison circuits are formed to detect load open circuit abnormality and load short circuit abnormality, and an inverter control circuit is controlled by the outputs from these two comparison circuits, thereby stopping the operation of the inverter. In a multi-lamp-type discharge lamp drive control circuit for a plurality of discharge lamps, abnormal operations are generally detected by forming two comparison circuits for detecting load open circuit abnormality and load short circuit abnormality for each discharge lamp.